1. Field of the Invention
The present invention relates to a dental implant that can be clinically applied in a simple and versatile manner with the jawbone serving as the anchor.
2. Description of the Related Art
As the general practice for dental implants that use the jawbone as the anchor, a hole (bone hole) with a diameter that is substantially the same as or slightly smaller than the outer diameter of a dental implant is formed in the jawbone using an implant drill, and then the dental implant is hammered or screwed into the bone hole so as to implant it. Thus the dental implant is implanted at a desired jawbone location.
Known examples of dental implants include the following: a two-piece type of dental implant that is constituted by two parts, namely a fixture serving as the dental root replacement and an abutment serving as the dental crown anchor replacement, with the abutment being joined to the fixture; a three-piece type of dental implant that is constituted by three parts, namely a fixture, an abutment, and a screw, with the abutment being fixed to the fixture using the screw; and a one-piece type of dental implant in which the fixture and the abutment are integrated.
Examples of dental implant procedures include the following: a two-part procedure in which the dental implant is implanted and the gum tissue is sutured such that the dental implant is not exposed inside the oral cavity, and then a healing period is allowed before a prosthesis (dental crown) is attached; and a one-part procedure in which the surrounding gum tissue is sutured so as to leave the implanted dental implant exposed.
The one-piece type of dental implant uses the simplest procedure and is advantageous in terms of physical strength as well. Also, the one-part procedure, in which the entire treatment is completed at one time by implanting the dental implant and attaching the occlusion site prosthesis serving as the upper structure immediately thereafter, is advantageous in terms of being able to shorten the total amount of time or number of days for the procedure and being able to reduce the financial and mental burden borne by the staff and patient. However, the one-piece type of dental implant and the one-part procedure only can be applied when conditions in the oral cavity are met, such as the condition that the bone material and bone mass of the jawbone are sufficient, the condition that the oral hygiene condition is favorable, and the condition that the procedure site and tooth alignment are such that the procedure site is not subjected to high occlusal pressure. The dental implant type and procedure have therefore been appropriately selected case-by-case.
Although recent years have seen advances in the development of technology for making dental implants easier to use, and products with improved surface texture, shape, and the like are being launched, these products all have advantages and disadvantages.
Japanese Patent No. 4278305 discloses a dental implant that can be easily implanted into a bone tissue structure. Formed on the outer face of this dental implant are two types of male threading that have the same or substantially the same lead (the amount of travel in the axial direction when turned one time, which is described as “pitch” in Japanese Patent No. 4278305) and different pitches (the distance between adjacent thread crests, which is described as “intercrest interval” in Japanese Patent No. 4278305). The dental implant of Japanese Patent No. 4278305 is intended to only facilitate implantation in a bone tissue structure, and does not give consideration to the accurate implantation of a dental implant at the location determined at the time of diagnosis.
Japanese Patent No. 4282479 is a patent held by the same person as the above-described Japanese Patent No. 4278305 and discloses a dental implant in which the portion that engages with the cortical bone tissue layer has a tapered shape such that the diameter increases toward the upper side. Japanese Patent No. 4282479 discloses that when this dental implant is screwed into the jawbone, an increase in torque can be recognized in a sensory manner with the tapered portion, thus enabling accurate implanting of the dental implant at the location determined at the time of diagnosis.
However, with the tapered portion in Japanese Patent No. 4282479 as well, the male threading is formed so as to continuously have the same lead and same pitch from the portion more toward the tip side. Accordingly, the amount of increase in the torque is constant when screwing in the tapered portion. Therefore, in reality, it is difficult to accurately implant the dental implant at the location determined at the time of diagnosis.
It has been confirmed that in many cases after the implantation of a dental implant, approximately 1 mm of bone resorption (regression) occurs from the bone surface toward the interior of the bone. One cause of this bone resorption is that bacteria intrudes from the bone surface side into the border between the bone and the dental implant, and the bone is eroded toward the interior of the bone along the border between the bone and the dental implant. According to reviews carried out by the inventors of the present invention, when microscopically viewed, this bone erosion does not advance in parallel toward the central axis of the dental implant, but rather advances along the roots of for example, the male threading formed on the outer surface of the dental implant. With the dental implant of Japanese Patent No. 4282479, the male threading is formed so as to continuously have the same lead and same pitch in the tapered portion and the portion farther toward the tip side, and therefore there is the problem that bone resorption caused by bacteria readily progresses along the roots of the male threading.